The present invention relates to a scintillator. More specifically, the present invention relates to a microcolumnar zinc selenide scintillator and use thereof, and methods of fabrication of microcolumnar scintillators using sublimation-based deposition techniques.
Scintillation spectrometers are widely used in detection and spectroscopy of energetic photons (e.g., X-rays and γ-rays). Such detectors are commonly used, for example, in nuclear and particle physics research, medical imaging, diffraction, non destructive testing, nuclear treaty verification and safeguards, nuclear non-proliferation monitoring, and geological exploration.
Important requirements for the scintillation materials used in these applications include high light output, transparency to the light it produces, high stopping efficiency, fast response, good proportionality, low cost and availability in large volume. These requirements are often not met by many of the commercially available scintillators. While general classes of chemical compositions may be identified as potentially having some attractive scintillation characteristic(s), specific compositions/formulations and structures having both scintillation characteristics and physical properties necessary for actual use in scintillation spectrometers and various practical applications, as well as capability of imaging at a high resolution, have proven difficult to predict or produce. Specific scintillation properties are not necessarily predictable from chemical composition alone, and preparing effective scintillators from even candidate materials often proves difficult. For example, while the composition of sodium chloride had been known for many years, the invention by Hofstadter of a high light-yield and conversion efficiency scintillator from sodium iodide doped with thallium launched the era of modern radiation spectrometry. More than half a century later, thallium doped sodium iodide, in fact, still remains one of the most widely used scintillator materials. Since the invention of NaI(Tl) scintillators in the 1940's, for half a century radiation detection applications have depended to a significant extent on this material. The fields of nuclear medicine, radiation monitoring and spectroscopy have grown up supported by NaI(Tl). Although far from ideal, NaI(Tl) was relatively easy to produce for a reasonable cost and in large volume. With the advent of X-ray CT in the 1970's, a major commercial field emerged as did a need for different scintillators, as NaI(Tl) was not able to meet the requirements of CT imaging. Later, the commercialization of PET imaging provided the impetus for the development of yet another class of detector materials with properties suitable for PET. As the methodology of scintillator development evolved, new materials have been added, and yet, specific applications, particularly those requiring high resolution imaging, are still hampered by the lack of scintillators suitable for particular applications.
As a result, there is continued interest in the search for new scintillator formulations and physical structures with both the enhanced performance and the physical characteristics needed for use in various applications. Today, the development of new scintillators continues to be as much an art as a science, since the composition of a given material does not necessarily determine its performance and structural properties as a scintillator, which are strongly influenced by the history (e.g., fabrication process) of the material as it is formed. While it is may be possible to reject a potential scintillator for a specific application based solely on composition, it is not possible to predict whether a material with promising composition will produce a scintillator with the desired properties.
One promising group of scintillator compositions includes those made of zinc selenide. Solid crystalline forms of doped ZnSe have been produced (e.g., doped with Hg, Cd, Te, or Zn). For example, bulk ZnSe crystals have been synthesized by melt-based techniques and using crystal growth techniques such as the traveling heater method (THM), though the high pressure Bridgman technique is known to successfully produced scintillation grade material. The latter method is complex, as it requires a specialized carbon fiber-lined growth furnace, high temperature heaters to reach ˜1600 C (needed for congruent melting of the material), and a well-balanced temperature gradient to foster nucleation and defect-free crystal growth. Furthermore, the process must be conducted in inert gas under high pressures of several atmospheres to ensure crystalline growth. Finally, an important step after crystal growth is annealing the crystals in a Zn atmosphere at very high temperatures to diffuse Zn in the bulk of the crystal, before the material can provide scintillation. The tremendous complexity of the current methods results in yield problems and a high cost for the scintillator.
Crystalline ZnSe thin films have also been fabricated by epitaxial growth techniques. However, very little or no work has been reported on the synthesis of efficient luminescent ZnSe (i.e., with dopants like Te) for X-ray/γ-ray detection. Additionally, this technique is limited to producing thin films, for example, measuring 1 μm or less in thickness, which can be inadequate for detecting X-rays due to inefficient absorption. Furthermore, this type of process is expensive and does not permit formation of a scintillator structure capable of both high detection efficiency and high-resolution imaging.
Besides crystalline form, ZnSe:Te can be synthesized in powder form, but powdered screens are limited in use due to a substantial tradeoff between detection efficiency, which increases with increasing scintillator thickness, and spatial resolution, which decreases with increasing scintillator thickness, inherent in the light diffusion process. Powdered screens have substantially decreased material density compared to non-powdered structures, thereby requiring much thicker screens needed to provide adequate photon (e.g., X-ray/γ-ray) absorption. Unfortunately, the much thicker powdered screens not only result in light loss due to internal scattering, but also result in poor spatial resolution. ZnSe material used in IR windows is manufactured by chemical vapor deposition (CVD), however, it does not include any activator doping and consequently is a non-scintillator.
Thus, a need exists for improved scintillator compositions and structures, including improved ZnSe scintillators, suitable for use in various radiation detection applications, including medical imaging applications, and capable of high resolution imaging.